As a conventional example of a double type coil for generating a slant magnetic field for MRI, a slant coil apparatus is disclosed in JPB No. 194842/1987. FIG. 8 is a plan development of the slant coil apparatus.
The slant coil apparatus 51 is constituted by an inner coil 2 having coil width of W52 and coil length of L52, and an outer coil 53 having coil width of W53 and coil length of L53. Each of current paths R52, R53 of the inner coil 52 and the outer coil 53 is of spiral shape. In the plan development state, each spiral of the current paths R52, R53 takes shape of winding clockwise. These current paths are formed, for example, by etching a conduction layer of a flexible print board. In relation that a current path part at the utmost inside of the spiral of the inner coil 52 is connected to a current path part at the utmost outside of the spiral of the outer coil 53, both coils 52, 53 are connected in series and current flows from the power source E in the arrow direction. Dash-and-dot lines P52, P53 show the center lines of the coil widths W52, W53.
Each of sides A52 and A53 and the center lines P52 and P53 are aligned, and the inner coil 52 and the outer coil 53 are spaced by a definite distance and overlaid, and then curved into two, inner and outer, nearly coaxial semi-cylinders with the coil widths W52, W53 being made semi-circumferential lengths respectively thereby a double type coil is formed. In such overlaid state, the inner coil 52 and the outer coil 53 are opposite to each other in the direction of the spiral of the current paths. Patterns of the current paths R52, R53 are formed, so that when current flows there, a slant magnetic field is generated in the inside space of the inner coil 52 and the leakage magnetic field is made approximately zero in the outside apace of the outer coil 53.
The coil width W52 and the coil length L52 of the inner coil 52 are, for example, 1036 mm and 640 mm respectively, and the coil width W53 and the coil length L53 of the outer coil 53 are, for example, 1224 mm and 710 mm respectively.
As shown in FIG. 9, four sets of the slant coil apparatuses 51 are mounted on circumferential surfaces of two inner and outer bobbins 55, 56 constituted in nearly coaxial cylinder shape in symmetry with respect to the center cross-section of the bobbin thereby a lateral slant coil apparatus for MRI is formed. The inner coil 52 is mounted on the inside bobbin 55 and the outer coil 53 is mounted on the outside bobbin 56 respectively, the sides A52, A53 being directed to the symmetry surface.
In the above-mentioned slant coil apparatus 51 in the prior art, the current paths R52, R53 of the inner coil 52 and the outer coil 53 are in spiral shape respectively, and each of the inner coil 52 and the outer coil 53 has a current path part to form a desired magnetic field (hereinafter referred to as "main path") and a current path part only to return a current (hereinafter referred to as "return path").
In FIG. 8, in the inner coil 52, a lower half from the center of the spiral of the spiral current path R52 is a main path and an upper half is a return path, and in the outer coil 53, an upper half from the center of the spiral of the spiral current path R53 is a main path and a lower half is a return path. Therefore, a problem exists in that the whole current path becomes long and the impedance becomes high uselessly, and the response speed becomes slow and the drive power is increased.
Also since the coil lengths L52, L53 become long and the electromagnetic force generated by the coil becomes large, a problem exists in that noise attendant on switching of the slant magnetic field increases.
Disclosure of Invention
An object of the present invention is to provide a double type coil where impedance can be lowered and coil length can be shortened.
According to the present invention, on double semi-circular cylinders having the axis commonly, a current path pattern comprising a current path in spiral shape and a plurality of current paths in horseshoe shape surrounding the spiral current path is formed respectively. Directions of the spirals on the two cylindrical surfaces are opposite to each other. In the plural horseshoe-shaped paths on the two cylindrical surfaces, a circuit is opened at the end on the same side of the cylinder. These current paths are all connected in series over the two surfaces thereby one current path is formed.